Embodiments of the invention relate generally to vehicle drive systems and, more specifically, to providing propulsion power from an auxiliary drive of a vehicle or non-vehicle system.
Electric vehicles and hybrid electric vehicles are typically powered by one or more energy storage devices, either alone or in combination with an internal combustion engine. In pure electric vehicles, the one or more energy storage devices powers the entire drive system, thereby eliminating the need for an internal combustion engine. Hybrid electric vehicles, on the other hand, include energy storage device power to supplement power supplied by an internal combustion engine, which greatly increases the fuel efficiency of the internal combustion engine and of the vehicle. Traditionally, the energy storage devices in electric or hybrid electric propulsion systems include batteries, ultracapacitors, flywheels, or a combination of these elements in order to provide sufficient energy to power an electric motor.
In some applications, an auxiliary drive is provided in addition to the propulsion system to operate auxiliary equipment. Such applications may include, for example, medium duty (MD) and heavy duty (HD) applications, including transit buses, trucks, light rail and other industrial equipment. Often, a reduction in cost, size, weight, and improved subsystem and component packaging plus improved system efficiency results by powering auxiliaries by a separate engine driven alternator or auxiliary power unit (APU) to produce electrical power to operate auxiliaries such as air conditioning components including Freon compressors, pumps, fans and heaters. In other potential applications the auxiliaries may be powered by a fuel cell. The electrical power produced may be alternating current (AC) power or direct current (DC) power. Motor driven auxiliary loads may be operated at a constant speed and frequency or operate at a variable speed by control of the electrical output frequency and voltage of the APU.
For additional control, subsystem components such as in an air conditioning unit, for example, are often operated in an on/off mode based on the level of temperature needed. Typically, the power rating of the APU is designed to handle the maximum load of all the auxiliary loads. Since not all auxiliary loads will be powered all the same time, typical APU systems operate at partial load with less than optimum efficiency.
Therefore, it is desirable to provide an electric and/or hybrid electric propulsion system allowing the excess power capability of the APU to be utilized to provide a portion of the propulsion load of the traction or propulsion drive system. In addition, it is desirable to direct a portion of the regenerative energy to power auxiliary loads during periods of deceleration instead of dissipating this energy in conventional brake systems. Alternatively, a portion of this regenerative energy can be used to partially recharge the energy storage system.